The present invention relates to a white balance adjusting device for a color video camera and, more particularly, to a white balance adjusting device suitably applicable to a video camera which detects color temperature information from a picture signal and performs correcting operation continuously during taking pictures.
A white balance adjusting device which adjust the white balance by using video signals provided by a pickup element is disclosed, for example, in National Technical Report, Vol. 31, No. 1, pp.98-102, Feburary 1985. This known white balance adjusting device is based on a principle that the mean value of video signals is substantially equivalent to a video signal obtained by taking a white object under the same lighting condition when the angle of view is sufficiently large.
Japanese Patent Publication No. 53-2290 discloses a white balance adjusting system which adjusts the white balance of an image on the basis of a decision that a portion of the image represented by a luminance signal of high level, for example, 95% of a rated luminance or above, is white.
Although the accuracy of white balance adjustment of such a device and system is practically high enough, it is possible that errors are produced depending on the color of the object in taking a chromatic object.
Another white balance adjusting device decides, as stated in Japanese Patent Publication No. 53-2290, that a portion of an object represented by red, blue and green signals is white, if each signal level is above a prederermined level, and adjusts the white balance of the image on the basis of the signals representing the portion regarded as being white.
Such a conventional technique, however, processes signals for white balance adjustment on the basis of a decision that the image is white even if the image is not apparently "white", when the object has a high chroma, namely, when a single chromatic color is taken fully in a screen. Consequently, correct color temperature correction is impossible and erroneous white balance adjustment results. Another white balance adjusting method has been proposed to eliminate such an erroneous white balance adjustment. According to this white balance adjusting method, a white balance adjusting device becomes operative only when an object is really "white". That is, this white balance adjusting device decides that the object is "white" only when all the respective levels of three chrominance signals, namely, red, blue and green signals, are higher than a predetermined level, for example, 90% of a rated level. That is, in most signal conditions in which the respective levels of the three signals is higher than 90% of the rated level the object is regarded as being white. Thus, this white balance adjusting method can effectively eliminate the foregoing disadvantages. However, since the criterion of judgement for deciding that an object is white is severe, the white balance can accurately be corrected by intentionally taking a white board only in taking an object in a studio under an appropriate lighting condition, but such a criterion is met rarely under an ordinary taking condition.
Prior arts relating to the present invention are disclosed in Japanese Patent Provisional Publication (Kokai) Nos. 55-2524, 58-142693 and 59-189793, and U.S. patent application Ser. No. 937,394: "White Balance adjusting Device of Color Video Camera", Toshio Murakami, Yasushi Takagi, Hiroyasu Otsubo and Yutaka Sato.